This invention relates generally to the field of optical fibers and more specifically to optical fibers which are hardened against the deteriorating effects of ionizing radiation on the optical transmissibility of the fiber.
Optical fibers finding substantial commercial use as optical data links in communication systems must necessarily be characterized by relatively low losses in optical transmissibility along the length of the fiber. To this end, considerable prior work has been done in the development of optical fibers having desirable combinations of refractive indices for the core and cladding characterized by transmission losses of less than 1 dB/m. The use of optical fibers as data links in certain environments, such as satellite systems, weapons or other military related equipment, certain medical diagnostic instrumentation, geological survey equipment, and the like may subject the fiber to the influence of ionizing nuclear radiation. Exposure to the ionizing radiation may temporarily or permanently disable the fiber optic core depending upon the radiation type, intensity and exposure time affecting the fiber optic core. Disablement of the core by the ionizing radiation is associated with the interaction of the radiation with impurities in the core material, certain of which impurities may be included to achieve the desirable refractive index for the core material. The interaction of the radiation with the impurities generates optical absorption sites which characteristically interfere with the transmissibility of the optical fiber. Substantial reduction of transmissibility may result under the influence of the ionizing radiation, although characteristically the core material may slowly recover some degree of transmissibility upon removal of the radiation. Shielding the fiber optics from the radiation using conventional materials is inappropriate in most cases because of the mass of material required for effective shielding.
Previous attempts to reduce the effects of the interaction of the optical fiber core with radiation have concentrated upon removing impurities from the core material or upon adding specific impurities to the core material to reduce the effectiveness of the optical absorption sites most affected by the radiation.
These troublesome optical absorption sites have been found to be susceptible to removal by optically exciting them with light of the proper wavelengths, usually either in the visible or in the ultraviolet range. The process of optically bleaching out the radiation affected absorption sites by light injection at a fiber end will not be effective for long fibers because the bleaching process is time dependent and occurs slowly enough that any desirable degree of bleaching down the fiber optic length is unacceptably delayed. Furthermore, most fibers inherently exhibit a higher degree of absorption at the shorter ultraviolet wavelengths which could be most effective for bleaching, than at the longer, near infrared, wavelengths, normally used for signal transmission, which further complicates the problem of bleaching the fiber core along its length.
The invention described herein provides a novel optical fiber configuration comprising a conventional optical fiber core, with a conventional cladding, which has an additional outer coating of phosphorescent or luminescent material, or a coating or sheath of a transparent material within which a suitable phosphor material is distributed. This outer coating or sheath material should have a refractive index equal to or less than the index of the cladding material. Upon exposure of the optical fiber structure of the present invention to ionizing radiation, the phosphor material will be stimulated by interaction with the radiation to emit light of a desirable ultraviolet or visible wavelength to bleach radiation-induced optical absorption centers in the core. The presence of the phosphor in the coating in close proximity to the fiber optic core and cladding layer along its entire length therefore provides a source of ultraviolet or visible light to bleach the core continuously during the exposure of the fiber to the radiation and during the short time immediately following the irradiation during which the phosphor continues to emit bleaching light with a characteristic decay time. An outer metallic coating may be applied to the structure to enhance internal reflections of the phosphorescent light.
It is therefore an object of this invention to provide an improved optical fiber data link.
It is a further object of this invention to provide an optical fiber which is partially hardened against the effects of ionizing nuclear radiation.
These and other objects of the present invention will become apparent as the detailed description of certain representative embodiments thereof proceeds.